/**
* acf/save_post filter to create custom sortable usernames field
*
* @param array $post_id
*/
function my_acf_save_post( $post_id ) {
// get users from ACF field
$my_users = get_field('my_users');
// get usernames
$usernames = array();
foreach($my_users as $my_user) {
$usernames[] = $my_user['nickname'];
}
$value = implode(", ", $usernames);
// add usernames to sortable field
update_post_meta($post_id, 'my_users_sortable', $value);
}
add_action('acf/save_post', 'my_acf_save_post', 20);